Chapter-9-Wires-and-cables_2005_Newnes-Electrical-Power-Engineer-s-Handbook-Second-Edition-

Prévia do material em texto

Chapter 9 
Wires and cables 
V.A.A. Banks and RH. Fraser 
With amendments by 
D. Gracias 
Pirelli Cables Ltd 
A.J. Willis 
Brand-Rex Ltd 
9.1 Scope 
Thousands of cable types are used throughout the world. They are found in applications 
ranging from fibre-optic links for data and telecommunication purposes through to EHV 
underground power transmission at 275 kV or higher. The scope here is limited to cover 
those types of cable which fit within the general subject matter of the pocket book. 
This chapter therefore covers cables rated between 300/500 V and 19/33 kV for use 
in the public supply network, in general industrial systems and in domestic and com-
mercial wiring. Optical communication cables are included in a special section. 
Overhead wires and cables, submarine cables and flexible appliance cords are not 
included. 
Even within this relatively limited scope, it has been necessary to restrict the cov-
erage of the major metallic cable and wire types to those used in the UK in order to 
give the cursory appreciation which is the main aim. 
9.2 Principles of power cable design 
9.2.1 Terminology 
The voltage designation used by the cable industry does not always align with that 
adopted by users and other equipment manufacturers, so clarification may be helpful. 
A cable is given a voltage rating which indicates the maximum circuit voltage for 
which it is designed, not necessarily the voltage at which it will be used. For example, 
a cable designated 0.6/1 kV is suitable for a circuit operating at 600 V phase-to-earth 
and 1000 V phase-to-phase. However it would be normal to use such a cable on dis-
tribution and industrial circuits operating at 230/400 V in order to provide improved 
safety and increased service life. For light industrial circuits operating at 230/400 V it 
would be normal to use cables rated at 450/750 V, and for domestic circuits operating 
at 230/400 V, cable rated at 300/500 V would often be used. Guidance on the cables 
that are suitable for use in different locations is given in BS 7540. 
248 Wires and cables 
The terms LV (Low Voltage), MV (Medium Voltage) and HV (High Voltage) have 
different meanings in different sectors of the electrical industry. In the power cable 
industry, the following bands are generally accepted and these are used in this chapter: 
LV- cable rated from 300/500 V to 1.9/3.3 kV 
MV- cables rated from 3.8/6.6 kV to 19/33 kV 
HV- cable rated at greater than 19/33 kV 
Multi-core cable is used in this chapter to describe power cable having two to five 
cores. Control cable having seven to 48 cores is referred to as multi-core control cable. 
Cable insulation and sheaths are variously described as thermoplastic, thermo-
setting, vulcanized, cross-linked, polymeric or elastomeric. All extruded plastic materi-
als applied to cable are polymeric. Those which would re-melt if the temperature 
during use is sufficiently high are termed thermoplastic. Those which are modified 
chemically to prevent them from re-melting are termed thermosetting, cross-linked or 
vulcanized. Although these materials will not re-melt, they will soften and deform at 
elevated temperatures, if subjected to excessive pressure. The main materials within 
the two groups are as follows: 
• thermoplastic 
- polyethylene (PE) 
- medium-density polyethylene (MDPE) 
- polyvinyl chloride (PVC) 
• thermosetting, cross-linked or vulcanized 
- cross-linked polyethylene (XLPE) 
- ethylene-propylene rubber (EPR) 
Elastomeric materials are polymeric. They are rubbery in nature, giving a flexible and 
resilient extrusion. Elastomers such as EPR are normally cross-linked. 
9.2.2 General considerations 
Certain design principles are common to power cables, whether they are used in the 
industrial sector or by the electricity supply industry. 
For many cable types the conductors may be of copper or aluminium. The initial 
decision made by a purchaser will be based on price, weight, cable diameter, avail-
ability, the expertise of the jointers available, cable flexibility and the risk of theft. 
Once a decision has been made, however, that type of conductor will generally then be 
retained by that user, without being influenced by the regular changes in relative price 
which arise from the volatile metals market. 
For most power cables the form of conductor will be solid aluminium, stranded alu-
minium, solid copper (for small wiring sizes) or stranded copper, although the choice 
may be limited in certain cable standards. Solid conductors provide for easier fitting 
of connectors and setting of the cores at joints and terminations. Cables with stranded 
conductors are easier to install because of their greater flexibility, and for some indus-
trial applications a highly flexible conductor is necessary. 
Where cable route lengths are relatively short, a multi-core cable is generally 
cheaper and more convenient to install than single-core cable. Single-core cables are 
sometimes used in circuits where high load currents require the use of large conductor 
sizes, between 500 mm^ and 1200 mm .̂ In these circumstances, the parallel connec-
tion of two or more multi-core cables would be necessary in order to achieve the 
required rating and this presents installation difficulties, especially at termination boxes. 
Newnes Electrical Power Engineer's Handbook 249 
Single-core cable might also be preferred where duct sizes are small, where longer 
cable runs are needed between joint bays or where jointing and termination require-
ments dictate their use. It is sometimes preferable to use 3-core cable in the main part 
of the route length, and to use single-core cable to enter the restricted space of a ter-
mination box. In this case, a transition from one cable type to the other is achieved 
using trifurcating joints which are positioned several metres from the termination box. 
Armoured cables are available for applications where the rigours of installation are 
severe and where a high degree of external protection against impact during service is 
required. Steel Wire Armour (SWA) cables are commonly available although Steel Tape 
Armour (STA) cables are also available. Generally, SWA is preferred because it enables 
the cable to be drawn into an installation using a pulling stocking which grips the out-
side of the oversheath and transfers all the pulling tension to the SWA. This cannot 
normally be done with STA cables because of the risk of dislocating the armour tapes 
during the pull. Glanding arrangements for SWA are simpler and they allow full usage 
of its excellent earth fault capability. In STA, the earth fault capability is much reduced 
and the retention of this capability at glands is more difficult. The protection offered 
against a range of real-life impacts is similar for the two types. 
9.2.3 Paper-insulated cables 
Until the mid-1960s, paper-insulated cables were used worldwide for MV power cir-
cuits. There were at that time very few alternatives apart from the occasional trial 
installation or special application using PE or PVC insulation. 
The position is now quite different. There has been a worldwide trend towards 
XLPE cable and the UK industrial sector has adopted XLPE-insulated or EPR-
insulated cable for the majority of MV applications, paper-insulated cable now being 
restricted to minor uses, such as extensions to older circuits or in special industrial 
locations. The use of paper-insulated cables for LV has been superseded completely by 
polymeric cables in all sectors throughout the world. 
The success of polymeric-insulated cables has been due to the much easier, cleaner 
and more reliable jointing and termination methods that they allow. However, because 
of the large amount of paper-insulated cable still in service and its continued specifi-
cation in some sectors, such as the regional public supply networks for MV circuits, 
its coverage here is still appropriate. 
Paper-insulated cables comprise copper or aluminium phaseconductors which are 
insulated with lapped paper tapes, impregnated with insulating compound and 
sheathed with lead, lead alloy or corrugated aluminium. For mechanical protection, 
lead or lead alloy sheathed cables are finished off with an armouring of steel tapes or 
steel wire and a covering of either bitumenized hessian tapes or an extruded PVC or 
PE oversheath. Cables which are sheathed with corrugated aluminium need no further 
metallic protection, but they are finished off with a coating of bitumen and an extruded 
PVC oversheath. The purpose of the bitumen in this case is to provide additional cor-
rosion protection should water penetrate the PVC sheath at joints, in damaged areas or 
by long-term permeation. 
There are, therefore, several basic types of paper-insulated cable and these are speci-
fied according to existing custom and practice as much as to meet specific needs and 
budgets. Particular features of paper-insulated cables used in the electricity supply indus-
try and in industrial applications are described in sections 9.3.1.1 and 9.3.2.1, respectively. 
The common element is the paper insulation itself. This is made up of many layers 
of paper tape, each applied with a slight gap between the turns. The purity and grade 
250 Wires and cables 
of the paper is selected for best electrical properties and the thickness of the tape is 
chosen to provide the required electrical strength. 
In order to achieve acceptable dielectric strength, all moisture and air is removed 
from the insulation and replaced by Mineral Insulating Non-Draining (MIND) com-
pound. Its waxy nature prevents any significant migration of the compound during the 
lifetime of the cable, even at full operating temperature. This is in contrast to oil-filled 
HV cables, utilizing a lower viscosity impregnant which must be pressurized through-
out the cable service life to keep the insulation fully impregnated. Precautions are 
taken at joints and terminations to ensure that there is no local displacement of MIND 
compound which might cause premature failure at these locations. The paper insula-
tion is impregnated with MIND compound during the manufacture of the cable, 
immediately before the lead or aluminium sheath is applied. 
A 3-core construction is preferred in most MV paper-insulated cables. The three 
cores are used for the three phases of the supply and no neutral conductor is included 
in the design. The parallel combination of lead or aluminium sheath and armour can 
be used as an earth continuity conductor, provided that circuit calculations prove its 
adequacy for this purpose. Conductors of 95 mm^ cross section and greater are sector-
shaped so that when insulated they can be laid up in a compact cable construction. 
Sector-shaped conductors are also used in lower cross sections, down to 35 mm ,̂ 
50 mm^ and 70 mm^ for cables rated at 6 kV, 10 kV and 15 kV, respectively. 
The 3-core 6.6 kV cables and most 3-core 11 kV cables are of belted design. The 
cores are insulated and laid up such that the insulation between conductors is adequate 
for the full line-to-line voltage (6.6 kV or 11 kV). The laid-up cores then have an addi-
tional layer of insulating paper, known as the belt layer, applied and the assembly is 
then lead sheathed. The combination of core insulation and belt insulation is sufficient 
for phase-to-earth voltage between core and sheath (3.8 kV or 6.35 kV). 
A 15 kV, 22 kV and 33 kV 3-core cables and some l lkV 3-core cables are of 
screened design. Here each core has a metallic screening tape and the core insulation 
is adequate for the full phase-to-earth voltage. The screened cores are laid up and the 
lead or aluminium sheath is then applied so that the screens make contact with each 
other and with the sheath. 
The bitumenized hessian serving or PVC oversheath is primarily to protect the 
armour from corrosion in service and from dislocation during installation. The PVC 
oversheath is now preferred because of the facility to mark cable details, and its clean 
surface gives a better appearance when installed. It also provides a smooth firm 
surface for glanding and for sealing at joints. 
9.2.4 Polymeric cables 
PVC and PE cables were being used for LV circuits in the 1950s and they started to 
gain wider acceptance in the 1960s because they were cleaner, lighter, smaller and 
easier to install than paper-insulated types. During the 1970s the particular benefits of 
XLPE and HEPR insulations were being recognized for LV circuits and today it is 
these cross-linked insulations, mainly XLPE, which dominate the LV market with 
PVC usage in decline for power circuits, although still used widely for low voltage 
wiring circuits. The LV XLPE cables are more standardized than MV polymeric types, 
but even so there is a choice of copper or aluminium conductor (circular or shaped), 
single-core or multi-core, SWA or unarmoured, and PVC or Low Smoke and Fume 
(LSF) sheathed. A further option is available for LV in which the neutral and/or earth 
conductor is a layer of wires applied concentrically around the laid-up cores rather 
Newnes Electrical Power Engineer's Handbook 251 
than as an insulated core within the cable. In this case, the concentric earth conductor 
can replace the armour layer as the protective metal layer for the cable. This concen-
tric wire design is mainly used in the LV electricity distribution network, whereas the 
armoured design is primarily used in industrial applications. 
Polyethylene and PVC were shown to be unacceptable for use as general MV cable 
insulation in the years following the 1960s because their thermoplastic nature resulted 
in significant temperature limitations. The XLFE and EPR were required in order 
to give the required properties. They allowed higher operating and short circuit 
temperatures within the cable, as well as the advantages of easier jointing and termi-
nating than for paper-insulated cables. This meant that in some applications a smaller 
conductor size could be considered than had previously been possible in the paper-
insulated case. 
The MV polymeric cables comprise copper or aluminium conductors insulated with 
XLFE or EPR and covered with a thermoplastic sheath of MDPE, PVC or LSF material. 
Within this general construction there are options of single-core or 3-core types, 
individual or collective screens of different sizes and armoured or unarmoured 
construction. Single-core polymeric cables are more widely used than single-core 
paper-insulated cables, particularly for electricity supply industry circuits. Unlike 
paper-insulated cables, MV polymeric 3-core cables normally have circular-section 
cores. This is mainly because the increases in price and cable diameter are usually 
outweighed in the polymeric case by simplicity and flexibility of jointing and termi-
nation methods using circular cores. 
Screening of the cores in MV polymeric cables is necessary for a number of rea-
sons, which combine to result in a two-level screening arrangement. This comprises 
extruded semiconducting layers immediately under and outside the individual XLFE 
or EPR insulation layer, and a metallic layer in contact with the outer semiconducting 
layer. The semiconducting layers are polymeric materials containing a high proportion 
of carbon, giving an electrical conductivity well below that of a metallic conductor, but 
well above that required for an insulating material. 
The two semiconducting layers must be in intimate contact in order to avoid partial 
discharge activity at the interfaces, where any minute air cavity in the insulation would 
cause a pulse of charge to transfer to and from the surface of the insulation in each 
half-cycle of applied voltage. These charge transfers result in erosion of the insulation 
surface and premature breakdown. In order to achieve intimate contact, the insulation 
and screens are extruded during manufacture as an integral triple layer and this is 
applied to the individual conductorin the same operation. The inner layer is known as 
the conductor screen and the outer layer is known as the core screen or dielectric 
screen. 
When the cable is energized, the insulation acts as a capacitor and the core screen 
has to transfer the associated charging current to the insulation on every half-cycle of 
the voltage. It is therefore necessary to provide a metallic element in contact with the 
core screen so that this charging current can be delivered from the supply. Without this 
metallic element, the core screen at the supply end of the cable would have to carry a 
substantial longitudinal current to charge the capacitance which is distributed along the 
complete cable length, and the screen at the supply end would rapidly overheat as a 
result of excessive current density. However the core screen is able to carry the current 
densities relating to the charging of a cable length of say 200 mm, and this allows the 
use of a metallic element having an intermittent contact with the core screen, or 
applied as a collective element over three laid-up cores. A 0.08-mm thick copper tape 
is adequate for this purpose. 
252 Wires and cables 
The normal form of armouring is a single layer of wire laid over an inner sheath of 
PVC or LSF material. The wire is of galvanized steel for 3-core cables and aluminium 
for single-core cables. Aluminium wire is necessary for single-core cables to avoid 
magnetizing or eddy-current losses within the armour layer. In unarmoured cable, the 
screen is required to carry the earth fault current resulting from the failure of any 
equipment being supplied by the cable or from failure of the cable itself. In this case, 
the copper tape referred to previously is replaced by a screen of copper wires of cross 
section between say 6 mm^ and 95 mm^, depending on the earth fault capacity of the 
system. 
9.2.5 Low Smoke and Fume (LSF) and fire performance cables 
Following a number of fire disasters in the 1980s, there has been a strong demand for 
cables which behave more safely in a fire. Cables have been developed to provide the 
following key areas of improvement: 
• improved resistance to ignition 
• reduced flame spread and fire propagation 
• reduced smoke emission 
• reduced acid gas or toxic fume emission 
An optimized combination of these properties is achieved in LSF cables, which provide 
all of the above-mentioned characteristics. 
The original concept of LSF cables was identified through the requirements of 
underground railways in the 1970s. At that time, the main concern was to maintain suf-
ficient visibility such that orderly evacuation of passengers through a tunnel could be 
managed if the power to their train were interrupted by a fire involving the supply 
cables. This led to the development of a smoke test known as the '3-metre cube', this 
being based on the cross section of a London Underground tunnel. This test is now 
defined in BS EN 50268. The reduced emissions of the toxic fumes also ensured pas-
sengers escape was not impaired. PVC sheathed cables can, by suitable use of highly 
flame retarded PVC materials, be designed to provide good resistance to ignition and 
flame spread, but they produce significant volumes of smoke and toxic fumes when 
burning. On this basis the LSF materials become specified for underground applica-
tions. The tests for reduced flame propagation are defined in BS EN 50265 (single 
cable) and BS EN 50266 (grouped cables), with tests for acid gas emission being 
defined in BS EN 50267. 
The demand for LSF performance has since spread to a wide range of products and 
applications and LSF now represents a generic family of cables. Each LSF cable will 
meet the 3-metre cube smoke emission, ignition resistance and acid gas emission tests, 
but fire propagation performance is specified as appropriate to a particular product 
and application. For instance, a power cable used in large arrays in a power station has 
very severe fire propagation requirements, while a cable used in individual short links 
to equipment would have only modest propagation requirements. Hence BS EN 50265 
would be appropriate to assess the single cable, but BS EN 50266 comprises of a 
number of categories to cater for varying numbers of cables grouped together. 
Additionally, there has been a significant growth in demand for cables that are 
expected to continue to function for a period in a fire situation, enabling essential ser-
vices to continue operation during the evacuation of buildings or during the initial fire 
fighting stages. In these cables, in addition to these LSF properties, the insulation is 
Newnes Electrical Power Engineer's Handbook 253 
expected to maintain its performance in a fire. This insulation may be achieved by use 
of a compacted mineral layer, mica/glass tapes or a ceramifiable silicone layer. Specific 
designs are described in section 9.3.3. 
9.3 Main classes of cable 
9.3.1 Cables for the electricity supply industry 
9.3.1.1 MV papeNnsulated cables 
Until the late 1970s, the large quantities of paper-insulated lead covered (PILC) cables 
used in the UK electricity supply industry for MV distribution circuits were manufac-
tured according to BS 6480. These cables also incorporated steel wire armour (SWA) 
and bitumenized textile beddings or servings. An example is illustrated in Fig. 9.1. The 
lead sheath provided an impermeable barrier to moisture and a return path for earth 
fault currents and the layer of SWA gave mechanical protection and an improved 
earth fault capacity. PILC cable continues to be specified by a few utilities; although 
conversion to XLFE designs are planned. 
Following successful trials and extensive installation in the early 1970s, a new 
standard (ESI 09-12) was issued in 1979 for Paper-Insulated Corrugated Aluminium 
Sheathed (PICAS) cable. This enabled the electricity supply industry to replace expen-
sive lead sheath and SWA with a corrugated aluminium sheath which offered a high 
degree of mechanical protection and earth fault capability, while retaining the proven 
reliability of paper insulation. The standard was limited to three conductor cross sections 
Fig. 9.1 Lead-sheathed paper-insulated MV cable for the electricity supply industry (courtesy of Pirelli 
Cables) 
254 Wires and cables 
Fig. 9.2 Paper-Insulated Corrugated Aluminium Sheathed (PICAS) cable for the electricity supply industry 
(courtesy of Pirelli Cables) 
(95 mm^, 185 mm^ and 300 mm^) using stranded aluminium conductors with belted 
paper insulation; although it later included designs with screened paper insulation. An 
example of PICAS cable is shown in Fig. 9.2. A PICAS cable was easier and lighter 
to install than its predecessor and it found almost universal acceptance in the UK elec-
tricity supply sector. It is still being specified by a few utilities, but as with PILC, a 
switch to XLPE designs is planned. 
9.3.1.2 MV polymeric cables 
High-quality XLPE cable has been manufactured for over 25 years. lEC 502 (revised 
in 1998 as lEC 60502) covered this type of cable and was first issued in 1975. A com-
parable UK standard BS 6622 was issued in 1986 and revised in 1999. 
The following features are now available in MV XLPE cables and these are 
accepted by the majority of users in the electrical utility sector: 
• copper or aluminium conductors 
• semiconducting conductor screen and core screen (which may be fully bonded 
or easily strippable) 
• individual copper tape or copper wire screens 
Newnes Electrical Power Engineer's Handbook 255 
• PVC, LSF or MDPE bedding 
• copper wire collective screens 
• steel wire or aluminium armour 
• PVC, LSF or MDPE oversheaths 
Early experience in North America in the 1960s resulted in a large number of premature 
failures, mainly because of poor cable construction and insufficient care in avoiding 
contamination of the insulation. The failures were due to water treeing, which 
is illustrated in Fig. 9.3. In the presence of water, ioniccontaminants and oxidation 
products, electric stress gives rise to the formation of tree-like channels in the XLPE 
insulation. These channels start either from defects in the bulk insulation (forming 
bow-tie trees) or at the interfaces between the semiconducting screens and the 
insulation (causing vented trees). Both forms of trees cause a reduction in electrical 
strength of the insulation and can eventually lead to breakdown. Water treeing has 
Fig. 9.3 Example of water treeing in a polymeric cable 
256 Wires and cables 
largely been overcome by better materials in the semiconducting screen and by 
improvements in the quality of the insulating materials and manufacturing techniques, 
and reliable service performance has now been established. 
The UK electricity supply industry gradually began to adopt XLPE-insulated or 
EPR-insulated cable for MV distribution circuits from the early 1990s in place of the 
PILC or PICAS cables. Each distribution company has specified the best construction 
for its particular needs. An example of the variation between companies is the differ-
ence in practice between sohdly bonded systems and the use of earthing resistors to 
limit the earth fault currents. In the former case, the requirement might typically be 
to withstand an earth fault current of 13 kA for three seconds. In the latter case, only 
1 kA for one second might be specified and the use of single-core cable with a copper 
wire screen in place of a 3-core cable with a collective copper wire screen or SWA is 
viable. Different cross-sectional areas of copper wire screen may be specified depend-
ing on the earth fault level in the intended installation network. The majority of specific 
designs being used by the UK electricity supply industry are now incorporated into BS 
7870-4.10 (for single core) or BS 7870-4.20 (for 3-core). Examples of XLPE cable 
designs being used or considered by the UK distribution companies are shown in 
Figs 9.4, 9.5 and 9.6. The latter shows the most commonly adopted design for 11 kV 
networks, with a similar design used for 33 kV networks although with stranded 
copper conductors due to the higher load transfer requirements. The MDPE sheaths 
are specified for buried installations, with LSF used for tunnel applications. 
Fig. 9.4 Example of lead-sheathed XLPE-insulated cable for use in the UK electricity supply industry 
(courtesy of Pirelli Cables) 
Newnes Electrical Power Engineer's Handbook 257 
Fig. 9.5 Example of 3-core SWA XLPE-insulated cable for use in the UK electricity supply industry (courtesy 
of Pirelli Cables) 
9.3.1.3 LV polymeric cables 
Protective Multiple Earthed (PME) systems which use Combined Neutral and Earth 
(CNE) cables have become the preferred choice in the UK pubhc supply network, both 
for new installations and for extensions to existing circuits. This is primarily because 
of the elimination of one conductor by the use of a common concentric neutral and 
earth, together with the introduction of new designs which use aluminium for all phase 
conductors. 
Before CNE types became established, 4-core paper-insulated sheathed and 
armoured cable was commonly used. The four conductors were the three phases and 
neutral, and the lead sheath provided the path to the substation earth. The incentive for 
PME was the need to retain good earthing for the protection of consumers. With the 
paper cables, while the lead sheath itself could adequately carry prospective earth fault 
currents back to the supply transformer, the integrity of the circuit was often jeopard-
ised by poor and vulnerable connections in joints and at terminations. By using the 
neutral conductor of the supply cable for this purpose the need for a separate earth 
conductor was avoided. 
The adoption of 0.6/1 kV cables with extruded insulation for underground public 
supply in the UK awaited the development of cross-linked insulation systems with a 
performance similar to paper-insulated systems in overload conditions. An example of the 
258 Wires and cables 
Fig. 9.6 Example of single-core copper wire screen XLPE-insulated cable for use in the UK electricity supply 
industry (courtesy of Pirelli Cables) 
cables which have been developed is the Waveform CNE type which is XLPE-insulated 
and has the neutral/earth conductor applied concentrically in a sinusoidal form. 
Insulated solid aluminium phase conductors are laid up to form a three-phase cable 
and the CNE conductor consists of a concentric layer of either aluminium or copper 
wires. 
If the wires in the CNE conductor are of aluminium, they are sandwiched between 
layers of unvulcanized synthetic rubber compound to give maximum protection 
against corrosion. This construction is known as Waveconal and is illustrated in Fig. 9.7. 
Where the CNE conductors are of copper, they are partially embedded in the rubber 
compound without a rubber layer over the wires. This is termed Wavecon and is illus-
trated in Fig. 9.8. Some electricity companies initially adopted Wavecon types because 
of concern over excessive corrosion in the aluminium CNE conductor, but through 
standardization all companies had moved to the use of copper wire design by 2001. 
Waveform cables are manufactured in accordance with BS 7870-3.4. 
Both waveform types are compact, with cost benefits. The aluminium conductors 
and synthetic insulation result in a cable that is light and easy to handle. In addition, 
the waveform lay of the CNE conductors enables service joints to be readily made 
without cutting the neutral wires, as they can be formed into a bunch on each side of 
the phase conductors. 
Newnes Electrical Power Engineer's Handbook 259 
Fig. 9.7 Construction of a 'Waveform' XLPE-insulated CNE cable; 'Waveconal' (courtesy of Pirelli Cables) 
9.3.2 Industrial cables 
'Industrial cables' are defined as those power circuit cables which are installed on the 
customer side of the electricity supply point, but which do not fall into the category of 
* wiring cables'. 
Generally these cables are rated 0.6/1 kV or above. They are robust in construction 
and are available in a wide range of sizes. They can be used for distribution of power 
around a large industrial site or for final radial feeders to individual items of plant. 
Feeder cables might be fixed or in cases, such as coal-face cutting machines and 
mobile cranes they may be flexible trailing or reeling cables. 
Many industrial cables are supplied to customers' individual specifications and since 
these are not of general interest they are not described here. The following sections 
focus on types which are manufactured to national standards and which are supplied 
through cable distributors and wholesalers for general use. 
9.3.2.1 Paper-insulated cables 
For ratings between 0.6/1 kV and 19/33 kV, paper-insulated cables for fixed installations 
were supplied in the UK to BS 480, and then to BS 6480 following metrication in 1969. 
These cables comprise copper or aluminium phase conductors insulated with lapped 
paper tapes, impregnated with MIND compound and sheathed with lead or lead alloy. 
For mechanical protection they were finished with an armouring of steel tapes or steel 
wire and a covering of bitumenized hessian tapes or an extruded PVC oversheath. 
260 Wires and cables 
Fig. 9.8 Construction of a 'Waveform' XLPE-insulated CNE cable; 'Wavecon' (courtesy of Pirelli Cables) 
The 3-core cables of this type with SWA have been preferred for most appHcations 
and these have become known as Paper-Insulated Lead-Covered Steel Wire Armoured 
(PILCSWA) cables. Single-core cables to BS 6480 do not have armouring; this is 
partly because the special installation conditions leading to the selection of single-core 
do not demand such protection and partly because a non-magnetic armouring, such as 
aluminium would be needed to avoid eddy current losses in the armour. These single-
core cables are known as Paper-Insulated Lead Covered (PILC). 
It has already been observedthat paper-insulated cables are now seldom specified 
for industrial use, but BS 6480 remains an active standard. 
9.3.2.2 Polymeric cables for fixed installations 
The XLPE-insulated cables manufactured to BS 5467 are generally specified for 
230/400V and 1.9/3.3 kV LV industrial distribution circuits. These cables have 
superseded the equivalent PVC-insulated cables to BS 6346 because of their higher 
current rating, higher short-circuit rating and better availabiUty. 
For MV applications in the range 3.8/6.6 kV to 19/33 kV, XLPE-insulated wire-
armoured cables to BS 6622 are usually specified. 
Newnes Electrical Power Engineer's Handbook 261 
Multi-core LV and MV cables are normally steel wire armoured. This armouring 
not only provides protection against impact damage for these generally bulky and 
exposed cables, but it is also capable of carrying very large earth fault currents and 
provides a very effective earth connection. 
Single-core cables are generally unarmoured, although aluminium wire armoured 
versions are available. Single-core cables are usually installed where high currents are 
present (for instance in power stations) and where special precautions will be taken to 
avoid impact damage. For LV circuits of this type, the most economic approach is to 
use unarmoured cable with a separate earth conductor, rather than to connect in parallel 
the aluminium wire armour of several single-core cables. For MV applications, 
each unarmoured cable has a screen of copper wires which would together provide an 
effective earth connection. 
Even in the harsh environment of coal mines, XLPE-insulated types are now 
offered as an alternative to the traditional PVC- and EPR-insulated cables used at 
LV and MV, respectively. In this application the cables are always multi-core types 
having a single or double layer of SWA. The armour has to have a specified minimum 
conductance because of the special safety requirements associated with earth faults 
and this demands the substitution of some steel wires by copper wires for certain cable 
sizes. 
Where LSF fire performance is needed, LV wire-armoured cables to BS 6724 
are the established choice. These cables are identical in construction and properties 
to those made to BS 5467 except for the LSF grade of sheathing material and the 
associated fire performance. Cables meeting all the requirements of BS 6724 and, in 
addition, having a measure of fire resistance such that they continue to function in a 
fire are standardized in BS 7846, further details of which are given in section 9.3.3. 
Similarly, BS 7835 for MV wire-armoured cable, which is identical to BS 6622 apart 
from the LSF sheath and fire performance, was issued in 1996 and revised in 2000. 
The only other type of standardized cable used for fixed industrial circuits is 
multi-core control cable, often referred to as auxiliary cable. Such cable is used to 
control industrial plant, including equipment in power stations. It is generally wire-
armoured and rated 0.6/1 kV Cables of this type are available with between 5 and 
48 cores. The constructions are similar to 0.6/1 kV power cables and they are manu-
factured and supplied to the same standards (BS 5467, BS 6346 and BS 6724, as 
appropriate). 
9.3.2.3 Polymeric cables for flexible connections 
Flexible connections for both multi-core power cables and multi-core control cables 
are often required in industrial locations. The flexing duty varies substantially from 
application to application. At one extreme a cable may need to be only flexible enough 
to allow the connected equipment to be moved occasionally for maintenance. At the 
other extreme the cable may be needed to supply a mobile crane from a cable reel or 
a coal-face cutter from cable-handling gear. 
Elastomeric-insulated and sheathed cable is used for all such applications. This may 
have flexible stranded conductors (known as 'class 5') or highly flexible stranded con-
ductors (known as 'class 6'). Where metallic protection or screening is needed, this 
comprises a braid of fine steel or copper wires. For many flexible applications the 
cable is required to have a resistance to various chemicals and oils. 
Although flexible cables will normally be operated on a 230/400 V supply, it is 
normal to use 450/750 V rated cables for maximum safety and integrity. 
262 Wires and cables 
A number of cable types have been standardized in order to meet the range of 
performance requirements and the specification for these is incorporated into BS 6500. 
Guidance on the use of the cables is provided in this standard and further information 
is available in BS 7540. 
9.3.3 Wiring cables 
The standard cable used in domestic and commercial wiring in the UK since the 1960s 
is a flat PVC twin-and-earth type, alternatively known as 6242Y cable. This comprises 
a flat formation of PVC-insulated live and neutral cores separated by a bare earth 
conductor, the whole assembly being PVC-sheathed to produce a flat cable rated at 
300/500 V. Cable is also available with three insulated cores and a bare earth, for use 
on double-switched lighting circuits. These forms of cable are ideal for installation 
under cladding in standard-depth plaster. They are defined in BS 6004, which covers 
a large size range, only the smaller sizes of which are used in domestic and commer-
cial circuits. 
There are other cable types included in BS 6004 which have more relevance to non-
domestic installations. These include cables in both flat and circular form, similar 
to the 6242Y type but with an insulated earth conductor. Circular cables designated 
6183Y are widely used in commercial or light industrial areas, especially where many 
circuits are mounted together on cable trays. Also included in BS 6004 are insulated 
conductors designated 649IX which are pulled into conduit or trunking in circuits 
where mechanical protection or the facility to re-wire are the key factors. 
Of recent years, LSF versions of the twin-and-earth cables and the conduit wires 
have become available and are being used in installations where particular emphasis 
on fire performance is required. Such cables are included in BS 7211. These are 
commonly known as 6242B (for twin flat) or 649IB (for single-core conduit wire). 
A significant change occurred in 2004 for all fixed wiring in electrical installations 
in the UK. An amendment was published to BS 7671 (the lEE Wiring Regulations) 
which specified new cable core colours to bring the UK more closely into line 
with practice in mainland Europe; the term harmonized core colours is often used. For 
single-phase fixed installations, the red phase and black neutral are replaced by brown 
phase and blue neutral, as used for many years in flexible cables for appliances. For 
three phase cables, the new phase colours are brown, black and grey instead of 
red, yellow and blue, with the neutral now blue instead of black. In both cases, the 
protective conductor is still identified by a green-yellow combination. An alternative 
for three-phase cables is for all phase cores to be brown and marking of LI, L2 and L3 
to be carried out at terminations. The neutral will be blue again in this case. Electrical 
installations commenced after 31 March 2004 may use either the new harmonized core 
colours or the pre-existing colours, but not both. New installations after 31 March 
2006 must only use the harmonized core colours. 
An alternative type of cable with outstanding impact and crush strength is mineral-
insulated cable (MICC) manufactured to BS EN 60702-1. This is often known by its 
trade name, Pyrotenax. In a MICC cable, the copper line and neutral conductors are 
positioned inside a copper sheath, the spaces between the copper components being 
filled with heavily compacted mineral powder of insulating grade. Pressure or impact 
applied to the cable merely compresses the powder in such a way that the insulation 
integrity is maintained. The copper sheath often acts asthe circuit earth conductor. An 
oversheath is not necessary but is often provided for reasons of appearance or for exter-
nal marking. An MICC cable has a relatively small cross section and is easy to install. 
Newnes Electrical Power Engineer's Handbook 263 
In shopping and office complexes or in blocks of flats there may be a need for a 
distribution sub-main to feed individual supply points or meters. If this sub-main is to 
be installed and operated by the owner of the premises, then a 0.6/1 kV split-concentric 
service cable to BS 4553 may be used. This comprises a phase conductor insulated in 
PVC or XLPE, around which is a layer of copper wires and an oversheath. Some of 
the copper wires are bare and these are used as the earth conductor. The remainder are 
polymer-covered and they make up the neutral conductor. For larger installations, 
3-core versions of this cable are available to manufacturers' specification. 
In circuits supplying equipment for fire detection and alarm, emergency lighting 
and emergency supplies, regulations dictate that the cables will continue to operate 
during a fire. This continued operation could be ensured by measures, such as embed-
ding the cable in masonry, but may be achieved by cables which are fire-resistant 
in themselves. BS 5839-1:2002, the code of practice for fire detection and fire alarm 
systems for buildings, recommends the use of fire resisting cables for mains power 
supply circuits and all critical signal paths in such systems. Fire-resistance tests for 
cables are set down in BS 6387, BS 8434-1, BS 8434-2 and EN 50200. The latter three 
tests are called up in BS 5839-1 although two levels of survival time are specified, 
30 minutes for 'standard' and 120 minutes for 'enhanced'. The three types of cable 
are recognized in the code of practice are to BS EN 60702-1 (as described earlier), 
BS 7846 and BS 7629 (both as described below). 
The Mice cables to BS EN 60702-1 should comply with the 'enhanced' perform-
ance, since the mineral insulation is unaffected by fire. An MICC cable will only fail 
when the copper conductor or sheath melts and where such severe fires might occur 
the cable can be sheathed in LSF material to assist in delaying the onset of melting. 
MICC cable is also categorized CWZ in BS 6387. 
A number of alternatives to MICC cables for fire resistance have been developed 
and standardized. Some rely on a filled silicone rubber insulation which degrades to 
an insulating char, which continues to provide separation between the conductors 
so that circuit integrity is maintained during a fire. Other types supplement standard 
insulation with layers of mica tape so that even if the primary insulation bums com-
pletely the mica tape provides essential insulation to maintain supplies during the fire. 
Both cable types are standardized in BS 7629, and in addition to complying with 
performance levels up to CWZ of BS 6387, designs also may comply with either the 
'standard' or 'enhanced' performance required by BS 5839-1. 
Some circuits requiring an equivalent level of fire resistance need to be designed 
with larger cables than are found in BS 7629. Such circuits might be for the main emer-
gency supply, fire-fighting lifts, sprinkler systems and water pumps, smoke extraction 
fans, fire shutters or smoke dampers. These larger cables are standardized in BS 7846, 
which includes the size range and LSF performance of BS 6724, but through the use 
of layers of mica tape to supplement the insulation these cables can be supplied to the 
CWZ performance level in BS 6387, and additionally to the 'standard' or 'enhanced' 
performance levels specified in BS 5839-1. An additional fire test category in BS 7846, 
called F3, may be considered to be more appropriate for applications where the cable 
might be subject to fire, impact and water spray in combination during the fire. 
9.4 Parameters and test methods 
There are a large number of cable and material properties which are controlled by 
the manufacturer in order to ensure fitness for purpose and reliable long-term service 
264 Wires and cables 
performance. However, it is the operating parameters of the finished installed cable 
which are of most importance to the user in cable selection. The major parameters of 
interest are as follows: 
• current rating 
• capacitance 
• inductance 
• voltage drop 
• earth loop impedance 
• symmetrical fault capacity 
• earth fault capacity 
These are dealt with in turn in the following sections. 
9.4.1 Current rating 
The current rating of each individual type of cable could be measured by subjecting 
a sample to a controlled environment and by increasing the load current passing 
through the cable until the steady-state temperature of the limiting cable component 
reached its maximum permissible continuous level. This would be a very costly 
way of establishing current ratings for all types of cable in all sizes, in all environments 
and in all ambient temperatures. Current ratings are therefore obtained using an 
internationally-accepted calculation method, published in lEC 60287. The formulae 
and reference material properties presented in lEC 60287 have been validated by 
correlation with data produced from laboratory experiments. 
Current ratings are quoted in manufacturers' literature and they are listed in lEE 
Wiring Regulations (BS 7671) for some industrial, commercial and domestic cables. 
The ratings are quoted for each cable type and size in air, in masonry, direct-in-ground 
and in underground ducts. Derating factors are given so that these quoted ratings can 
be adjusted for different environmental conditions, such as ambient temperature, soil 
resistivity or depth of burial. 
Information is given in BS 7671 and in the lEE Guidance Notes on the selection of 
the appropriate fuse or mcb to protect the cable from overload and fault conditions, and 
general background is given in sections 8.2 and 8.3. 
9.4.2 Capacitance 
The capacitance data in manufacturers' literature is calculated from the cable dimen-
sions and the permittivity of the insulation. 
For example, the star capacitance of a 3-core belted armoured cable to BS 6346 
is the effective capacitance between a phase conductor and the neutral star point. It is 
calculated using the following formula: 
C = ^ ( |LiF/km) (Q iA 
where e^ = relative permittivity of the cable insulation (8.0 for PVC) 
d = diameter of the conductor (mm) 
ti = thickness of insulation between the conductors (mm) 
2̂ = thickness of insulation between conductor and armour (mm) 
Newnes Electrical Power Engineer's Handbook 265 
Equation 9.1 assumes that the conductors are circular in section. For those cables 
having shaped conductors, the value of capacitance is obtained by multiplying the figure 
obtained using eqn 9.1 by an empirical factor of 1.08. 
The calculated capacitance tends to be conservative, that is the actual capacitance 
will always be lower than the calculated value. However, if an unusual situation arises 
in which the cable capacitance is critical, then the manufacturer is able to make a 
measurement using a capacitance bridge. 
If the measured capacitance between cores and between core and armour is quoted, 
then the star capacitance can be calculated using eqn 9.2: 
9C -C 
C= '^ ' (|iiF/km) (9.2) 
where Q = measured capacitance between one conductor and the other 
two connected together to the armour 
Cy= measured capacitance between three conductors connected 
together and the armour 
9.4.3 inductance 
The calculation of cable inductance L for the same example of a 3-core armoured cable 
to BS 6346 is given by eqn 9.3 as follows: 
L = 1.02 X {0.2 X ln[2F/J] + k} (mH/km) (9.3) 
where d = diameter of the conductor (mm) 
Y = axial spacing between conductors (mm) 
/: = a factor which depends on the conductor make-up 
{k = 0.064 for 7-wire stranded 
0.055 for 19-wire stranded 
0.053 for 37-wire stranded 
0.050 for soUd) 
The same value of cable inductanceL is used for cables with circular- or sector-shaped 
conductors. 
9.4.4 Voltage drop 
BS 7671 specifies that within customer premises the voltage drop in cables is to be a 
maximum value of 4 per cent. It is therefore necessary to calculate the voltage drop 
along a cable. 
The cable manufacturer calculates voltage drop assuming that the cable will be 
loaded with the maximum allowable current which results in the maximum allowable 
operating temperature of the conductor. The cable impedance used for calculating the 
voltage drop is given by eqn 9.4. 
Z={R^ + {iKfL - l/lnfCyy^^ (Q/m) (9.4) 
where i? = ac resistance of the conductor at maximum conductor 
temperature (Q/m) 
L = inductance (H/m) 
C = capacitance (F/m) 
/ = supply frequency (Hz) 
266 Wires and cables 
The voltage drop is then given by eqns 9.5 and 9.6: 
For single-phase circuits: 
voltage drop = 2Z (V/A/m) (9.5) 
and for three-phase circuits: 
voltage drop = {1>TZ (V/A/m) (9.6) 
9.4.5 Symmetrical and earth fault capacity 
It is necessary that cables used for power circuits are capable of carrying any fault cur-
rents that may flow, without damage to the cable; the requirements are specified in BS 
7671. This assessment demands a knowledge of the maximum prospective fault cur-
rents on the circuit, the clearance characteristics of the protective device (as explained 
in Chapter 8) and the fault capacity of the relevant elements in the cable. For most 
installations it is necessary to establish the let-through energy of the protective device 
and to compare this with the adiabatic heating capacity of the conductor (in the case of 
symmetrical and earth faults) or of the steel armour (in the case of earth faults). 
The maximum let-through energy (Ih) of the protective device is explained 
in Chapter 8. It can be obtained from the protective device manufacturer's data. 
In practice the value will be less than that shown by the manufacturer's information 
because of the reduction in current during the fault which results from the significant 
rise in temperature and resistance of the cable conductors. 
The fault capacity of the cable conductor and armour can be obtained from 
information given in BS 7671 and the appropriate BS cable standard, as follows: 
]^S^ = adiabatic fault capacity of the cable element (9.7) 
where S = the nominal cross section of the conductor or 
the nominal cross section of, say, the armour (mm^) 
k = a. factor reflecting the resistivity, temperature coefficient, allowable 
temperature rise and specific heat of the metallic cable element 
(k= 115 for a PVC-insulated copper conductor within the cable 
= 176 for an XLPE-insulated copper earth conductor external to 
the cable 
= 46 for the steel armour of an XLPE-insulated cable) 
In practice it will be found that provided the cable rating is at least equal to the 
nominal rating of the protective device and the maximum fault duration is less than 
5 seconds, the conductors and armour of the cables to BS will easily accommodate the 
let-through energy of the protective device. 
It is also important that the impedance of the supply cable is not so high that 
the protective device takes too long to operate during a zero-sequence earth fault on 
connected equipment. This is important because of the need to protect any person 
in contact with the equipment, by limiting the time that the earthed casing of the equip-
ment, say, can become energized during an earth fault. 
This requkement, which is stated in BS 7671, places restrictions on the length of the 
cable that can be used on the load side of a protective device, and it therefore demands 
knowledge of the earth fault loop impedance of the cable. Some cable manufacturers have 
calculated the earth fault impedance for certain cable types and the data are presented in 
specialized hterature. These calculations take account of the average temperature of each 
conductor and the reactance of the cable during the fault. The values are supported by 
Newnes Electrical Power Engineer's Handbook 267 
independent experimental results. BS 7671 allows the use of such manufacturer's data or 
direct measurement of earth fault impedance on a completed installation. 
9.5 Optical communication cables 
The concept of using light to convey information is not new. There is a historical evi-
dence that Aztecs used flashing mirrors to conmiunicate and in 1880 Alexander 
Graham Bell first demonstrated his photophone, in which a mirror mounted on the end 
of a megaphone was vibrated by the voice to modulate a beam of sunlight, thereby 
transmitting speech over distances up to 200 m. 
Solid-state photodiode technology has its roots in the discovery of the light-sensitive 
properties of selenium in 1873, used as the detector in Bell's photophone. The Light 
Emitting Diode (LED) stems from the discovery in 1907 of the electroluminescent 
properties of silicon, and when the laser was developed in 1959, the components of an 
optical communication system were in place, with the exception of a suitable trans-
mission medium. 
The fundamental components of a fibre optic system are shown in Fig. 9.9. This 
system can be used for either analogue or digital transmissions, with a transmitter 
which converts electrical signals into optical signals. The optical signals are launched 
through a joint into an optical fibre, usually incorporated into a cable. Light emitting 
from the fibre is converted back into its original electrical signal by the receiver. 
9.5.1 Optical fibres 
An optical fibre is a dielectric waveguide for the transmission of light, in the form of 
a thin filament of very transparent silica glass. As shown in Fig. 9.10, a typical fibre 
comprises a core, the cladding, a primary coating and sometimes a secondary coating 
or buffer. Within this basic construction, fibres are further categorized as multi-mode 
or single-mode fibres with a step or graded index. 
The core is the part of the fibre which transmits light, and it is surrounded by a glass 
cladding of lower refractive index. In early fibres, the homogeneous core had a con-
stant refractive index across its diameter, and with the refractive index of the cladding 
also constant (at a lower value) the profile across the whole fibre diameter (as shown 
in Fig. 9.11(a)) became known as a step index. In this type of fibre, the light rays can 
be envisaged as travelling along a zigzag path of straight lines, kept within the core by 
total reflection at the inner surface of the cladding. Depending on the angle of the rays 
to the fibre axis, the path length will differ so that a narrow pulse of light entering the 
fibre will become broader as it travels. This sets a limit to the rate at which pulses can 
be transmitted without overlapping and hence a limit to the operating bandwidth. 
information input| 
(eiectricai) 
Transmitter 
Connector 
Light 
Connector 
Receiver 
Optical fibre 
Infomnation 
output 
(optical) 
Fig. 9.9 Basic fibre optic system 
268 Wires and cables 
Fig. 9.10 Basic optical fibre 
To minimize this effect, which is known as mode dispersion, fibres have been 
developed in which the homogeneous core is replaced by one in which the refractive 
index varies progressively from a maximum at the centre to a lower value at the 
interface with the cladding. Figure 9.11(b) shows such a graded index fibre, in which 
the rays no longer follow straight lines. When they approach the outer parts of the core, 
travelling temporarily faster, they are bent back towards the centre where they travel 
more slowly. Thus the more oblique rays travel faster and keep pace with the slower 
rays travelling nearer the fibre centre. This significantly reduces the pulse broadening 
effect of step index fibres. 
The mode dispersion of step index fibres has also been minimized by the development 
of single-mode fibres. As shown in Fig. 9.11(c), although it is a step index fibre, the 
core is so small (of the order of 8 |Lim in diameter) that only onemode can propagate. 
Fibre manufacture involves drawing down a preform into a long thin filament. 
The preform comprises both core and cladding, and for graded index fibres, the core 
contains many layers with dopants being used to achieve the varying refractive index. 
Although the virgin fibre has a tensile strength comparable to that of steel, its strength 
is determined by its surface quality. Microcracks develop on the surface of a virgin 
fibre in the atmosphere, and the lightest touch or scratch makes the fibre impractically 
fragile. Thus it must be protected, in line with the glass drawing before it touches 
any solid object, such as pulleys or drums, by a protective coating of resin, acetate or 
plastic material, known as the primary coating. 
Typically the primary coating has a thickness of about 60 |xm, and in some cases a 
further layer of material called the buffer is added to increase the mechanical protection. 
Another type of optical fibre has a plastic construction, with either step or graded 
index cores. Although larger in size (up to 1.0 mm cladding diameter) and with higher 
transmission losses than glass fibres, plastic optical fibres have economic and handling 
advantages for short distance, low data rate communication systems. 
9.5.2 Optical cable design 
The basic aim of a transmission cable is to protect the transmission medium from 
its environment and the rigours of installation. Conventional cables with metallic 
conductors are designed to function effectively in a wide range of environments, as 
N
ew
nes E
lectrical P
ow
er E
ngineer's H
andbook 
269 
03 
E
 
y 
V
 
c 
•1 iS 
O
 
.2 o 
O
 
E
 
=1. 
s 
•o
 
t CO 
V A 
/' 
^ 
O
) 
c 
O
 
2 o 
o
 
E
 
^ 
c o 
X
 
f — 
©
 
8
| 
ii 
o
 
t>0 
^ OS 
270 Wires and cables 
mentioned in sections 9.2 and 9.3. However, optical fibres differ significantly from 
copper wires to an extent that has a considerable bearing on cable designs and 
manufacturing techniques. The transmission characteristics and lifetime of fibres are 
adversely affected by quite low levels of elongation, and lateral compressions can 
produce small kinks or sharp bends which create an increase in attenuation loss known 
as microbending loss. This means that cables must protect the fibre from strain during 
installation and service, and they must cater for longitudinal compression that occurs, 
for example with a change in cable temperature. 
Fibre life in service is influenced by the presence of moisture as well as stress. 
The minute cracks which cover the surface of all fibres can grow if the fibre is stressed 
in the presence of water, so that the fibre could break after a number of years in 
service. Cables must be able to provide a long service life in such environments as 
tightly packed ducts which are filled with water. 
The initial application of optical cables was the trunk routes of large telecommuni-
cations networks, where cables were directly buried or laid in ducts in very long 
lengths, and successful cable designs evolved to take into account the constraints 
referred to earlier. The advantages of fibre optics soon led to interest in other applica-
tions, such as computer and data systems, premises cabling, military systems and 
industrial control. This meant that cable designs had to cater for tortuous routes of 
installation in buildings, the flexibiUty of patch cords and the arduous environments 
of military and industrial applications. Further opportunities for optical cables are pre-
sented by installations in existing rights-of-way, such as sewers, gas pipes and water 
lines without the need for costly civil engineering works. 
Nevertheless, many of the conventional approaches to cable design can be used 
for optical cables, with modification to take into account the optical and mechanical 
characteristics of fibres and their fracture mechanics. 
Cables generally comprise several elements or individual transmission components, 
such as copper pairs, or one or more optical fibres. The different types of element used 
in optical cables are shown in Fig. 9.12. 
The primary-coated fibre can be protected by a buffer of one or more layers of 
plastic material as shown in Fig. 9.12(a). Typically for a two-layer buffer, the inner 
layer is of a soft material acting as a cushion with a hard outer layer for mechanical 
protection, the overall diameter being around 850 fxm. In other cases, the buffer can be 
applied with a sliding fit to allow easy stripping over long lengths. 
In ruggedized fibres further protection for a buffered fibre is provided by surround-
ing it with a layer of non-metallic synthetic yams and an overall plastic sheath. 
This type of arrangement is shown in Fig. 9.12(b). 
When one or more fibres are run loosely inside a plastic tube, as shown in Fig. 9.12(c), 
they can move freely and will automatically adjust to a position of minimum bending 
strain to prevent undue stress being applied when the cable is bent. If the fibre is 
slightly longer than the tube, a strain margin is achieved when the cable is stretched, 
say during installation, and for underground and duct cables the tube can be filled with 
a gel to prevent ingress of moisture. Correct choice of material and manufacturing 
technique can ensure that the tube has a coefficient of thermal expansion similar to that 
of the fibre, so that microbending losses are minimized with temperature excursions. 
Optical fibres can be assembled into a linear array as a ribbon, as shown in 
Fig. 9.12(d). Up to 12 fibres may be bonded together in this way or further encapsu-
lated if added protection is required. 
In order to prevent undue cable elongation which could stress the fibres, optical 
cables generally incorporate a strength member. This may be a central steel wire or 
N
ew
nes E
lectrical P
ow
er E
ngineer's H
andbook 
271 
c o 
n .3 £1 
C
Q
 
U
 
CO
 
u 
272 Wires and cables 
strand, or non-metallic fibreglass rods or synthetic yams. The strength member should 
be strong, light and usually flexible, although in some cases a stiff strength member 
can be used to prevent cable buckling which would induce microbending losses in the 
fibres. Strength members are shown in the cable layouts in Fig. 9.13(b) and Fig. 9.13(c). 
The strength member can be incorporated in a structural member which is used 
as a foundation for accommodating the cable elements. An example is shown in 
Fig. 9.13(c), where a plastic section with slots is extruded over the strength member 
with ribbons inserted into the slots to provide high fibre count cables. 
A moisture barrier can be provided either by a continuous metal sheath or by a 
metallic tape with a longitudinal overlap, bonded to the sheath. Moisture barriers can 
be of aluminium, copper or steel and they may be flat or corrugated. In addition, other 
cable interstices may be filled with gel or water-swellable filaments to prevent the lon-
gitudinal ingress of moisture. 
Where protection from external damage is required, or where additional tensile 
strength is necessary, armouring can be provided; this may be metallic or non-metalUc. 
For outdoor cables, an overall sheath of polyethylene is applied. For indoor cables the 
sheath is often of low-smoke zero-halogen materials for added safety in the event of fire. 
Although the same basic principles of cable construction are used, the wide range 
of applications result in a variety of cable designs, from simplex indoor patch cords to 
cables containing several thousand fibres for arduous environments, to suboceanic 
cables. Figure 9.13 shows just a few examples. 
9.5.3 Interconnections 
The satisfactory operation of a fibre optic system requires effective jointing and ter-
mination of the transmission medium in the form of fibre-to-fibre splices and fibre 
connections to repeaters and end equipment. This is particularly important because 
with very low loss fibres the attenuation due to interconnections canbe greater than 
that due to a considerable length of cable. 
For all types of interconnection there is an insertion loss, which is caused by Fresnel 
reflection and by misalignment of the fibres. Fresnel reflection is caused by the changes 
in refractive index at the fibre-air-fibre interface, but it can be minimized by inserting 
into the air gap an index-matching fluid with the same refractive index as the core. 
Misalignment losses arise from three main sources as shown in Fig. 9.14. Inter-
connection designs aim to minimize these losses. End-face separation (Fig. 9.14(a)) 
allows light from the launch fibre to spread so that only a fraction is captured by the 
receive fibre; this should therefore be minimized. Normally the fibre cladding is used 
as the reference surface for aligning fibres, and the fibre geometry is therefore impor-
tant, even when claddings are perfectly aligned. Losses due to lateral misalignment 
(Fig. 9.14(b)) will therefore depend on the core diameter, non-circularity of the core, 
cladding diameter, non-circularity of the cladding and the concentricity of the core and 
cladding in the fibres to be jointed. Angular misalignment can result in light entering 
the receive fibre at such an angle that it cannot be accepted. It follows that very close 
tolerances are required for the geometry of the joint components and the fibres to be 
jointed, especially with single-mode fibres with core diameters of 8 |im and cladding 
diameters of 125 jiim. 
The main types of interconnections are fibre splices and demountable connectors. 
Fibre splices are permanent joints made between fibres or between fibres and 
device pigtails. They are made hy fusion splicing or mechanical alignment. In fusion 
splicing, prepared fibres are brought together, aligned and welded by local heating 
N
ew
nes E
lectrical P
ow
er E
ngineer's H
andbook 
273 
274 Wires and cables 
(a) End face separation 
(b) Lateral misalignment 
(c) Angular misalignment 
Fig. 9.14 Sources of misalignment loss 
combined with axial pressure. Sophisticated portable equipment is used for fusion 
splicing in the field. This accurately aligns the fibres by local light injection and it 
carries out the electric arc welding process automatically. Nevertheless, a level of skill 
is required in the preparation of the fibres, stripping the buffers and coatings and 
cleaving the fibres to achieve a proper end face. There are a number of mechanical 
techniques for splicing fibres which involve fibre alignment by close tolerance tubes, 
ferrules and v-grooves, and fixing by crimps, glues or resins. Both fusion and mechani-
cal splicing techniques have been developed to allow simultaneous splicing of fibres 
which are particularly suitable for fibre ribbons. For a complete joint, the splices must 
be incorporated into an enclosure which is suitable for a variety of environments, such 
as underground chambers or pole tops. The enclosure must also terminate the cables 
and organize the fibres and splices, and cassettes are often used where several hundred 
splices are to be accommodated. 
Demountable connectors provide system flexibility, particularly at and within the 
transmission equipment and distribution panels, and they are widely used on patch 
cords in certain data systems. As with splices, the connector must minimize Fresnel 
and misalignment loss, but it must also allow for repeated connection and disconnec-
tion, it must protect the fibre end face and it must cater for mechanical stress* such as 
tension, torsion and bending. There are many designs but in general the tolerances 
which are achievable on the dimensions of the various components result in a higher 
optical loss than in a splice. Demountable connectors have also been developed for 
Newnes Electrical Power Engineer's Handbook 275 
multiple-fibre simultaneous connection, with array designs being particularly suitable 
for fibre ribbons. For connector-intensive systems, such as office data systems, use is 
made of factory-predetermined cables and patch cords to reduce the need for on-site 
termination. 
9.5.4 Installation 
Optical fibre cables are designed so that normal installation practices and equipment 
can be used wherever possible, but as they generally have a lower strain limit than 
metallic cables special care may be needed in certain circumstances and manufacturer's 
recommendations regarding tensile loads and bending radii should be followed. 
Special care may be required in the following circumstances: 
• because of their light weight, optical cables can be installed in greater lengths 
than metallic cables. For long underground ducts access may be needed at inter-
mediate points for additional winching effort and space should be allowed for 
larger Tigure 8' cable deployment. 
• mechanical fuses and controlled winching may be necessary to ensure that the 
rated tensile load is not exceeded 
• guiding equipment may be necessary to avoid subjecting optical cables to 
unacceptable bending stresses, particularly when the cable is also under tension 
• when installing cables in trenches the footing should be free from stones. These 
could cause microbending losses. 
• in buildings, and particularly in risers, cleats and fixings should not be 
overtightened, or appropriate designs should be used to prevent compression 
and the resulting microbending losses 
• indoor cable routes should provide turning points if a large number of bends is 
involved. Routes should be as straight as possible. 
• excess lengths for jointing and testing of optical cables are normally greater that 
those required for metallic cables 
• where non-metallic optical cables are buried, consideration of the subsequent 
location may be necessary. Marker posts and the incorporation of a location 
wire may be advisable. 
Blown fibre systems have been developed as a means of avoiding fibre overstrain for 
complex route installation and of allowing easy system upgrading and future proofing. 
It results in low initial capital costs and provides for the distribution of subsequent 
costs. Initially developed by British Telecom, the network infrastructure is created by 
the most appropriate cabling method, being one or a group of empty plastic tubes. As 
and when circuit provision is required, one or more fibres can be blown by compressed 
air into the tubes. Individual tubes can, by means of connectors, be extended within 
buildings up to the fibre terminating equipment. The efficient installation of fibres into 
the tube network often requires the use of specially-designed fibres and equipment, 
such as air supply modules, fibre insertion tools and fibre pay-offs. For installation it is 
necessary to follow the instructions provided by the supplier, taking into account the 
requirements for the use of portable electrical equipment and compressed air, and the 
handling, cutting and disposal of optical fibres. A novel variation of this system is a data 
cable used for structural wiring systems. In a Tigure-8' configuration one unit comprises 
a 4-pair data cable and the other an empty tube, so that when an upgrade is required to 
an optical system the appropriate fibre can be blown in without the need for recabling. 
276 Wires and cables 
9.6 Standards 
9.6.1 Metallic wires and cables 
Most generally available cables are manufactured to recognized standards which may 
be national, European or international. Each defines the construction, the type and 
quality of constituent materials, the performance requirements and the test methods for 
the completed cable. 
The lEC standards cover those cables which need to be standardized to facilitate 
world trade, but this often requires a compromise by the parties involved in the 
preparation and acceptance of a standard. Where cables are to be used in a particular 
country, the practices and regulations in that country tend to encourage the more 
specific cable types defined in the national standards for that country.BS remains the 
most appropriate for use in the UK, and for the main cable types described in this 
chapter reference has therefore been made mainly to the relevant BS. 
Some cables rated at 450/750 V or less have through trade become standard 
throughout the EU, and these have been incorporated into Harmonization Documents 
(HDs). Each EU country must then pubhsh these requirements within a national 
standard. A harmonized cable type in the UK for instance would still be specified to 
the relevant BS and the cable would, if appropriate, bear the <HAR> mark. 
The key standards for metallic wires and cables which have been referred to in the 
chapter are listed in Table 9.1. 
Table 9,1 International and national standards for metallic wires and cables 
lEC HD BS Subject 
60502-1 
60227 
60245 
60055 
60227 and 
60245 
60502-2 
603 
21 
22 
621 
621 
21 and 
22 
620 
4553 
5467 
6004 
6007 
6346 
6387 
6480 
(EA 09-12) 
6500 
6622 
6724 
22 7211 
60364 
7629 
7671 
600/1000 V PVC-insulated single-phase split concentric 
cables with copper conductors 
Cables with thermosetting insulation up to 600/1000 V 
and up to 1900/3300 V 
Non-armoured PVC-insulated cables rated up to 450/750 V 
Non-armoured rubber-insulated cables rated up to 450/750 V 
PVC-insulated cables 
Performance requirements for cables required to 
maintain integrity under fire conditions 
Impregnated paper-Insulated lead sheathed cables up to 
33 000 V 
Paper-insulated corrugated aluminium sheathed 
6350/11000 V cable 
Insulated flexible cords and cables rated up to 450/750 V 
Cables with XLPE or EPR insulation from 3800/6600 V 
up to 19 000/33 000 V 
600/1000 V and 1900/3300 V armoured cables having 
thermosetting insulation with low emission of smoke 
and corrosive gases in fire 
Non-armoured cables having thermosetting insulation 
rated up to 450/750 V with low emission of smoke and 
corrosive gases in fire 
Fire resistant thermosetting insulated cables rated at 
300/500 V with limited circuit Integrity in fire 
Requirements for electrical installations: lEE Wiring 
Regulations (16th edition) 
Newnes Electrical Power Engineer's Handbook 277 
Table 9.1 (contd) 
lEC HD BS Subject 
60287 7769 Electric cables - calculation of current rating 
7835 Cables with XLPE or ERR insulation from 3800/6600 V 
up to 19 000/33 000 V with low emission of smoke and 
corrosive gases in fire 
603 7870-3.40 Polymeric insulated cables for distribution rated at 600/1000 V 
620 7870-4.10 Polymeric insulated cables for distribution rated from 
3800/6600V up to 19000/33000 V: single-core cable 
with copper wire screens 
620 7870-4.20 Polymeric insulated cables for distribution rated from 
3800/6600 V up to 19 000/33 000 V: 3 core cable with 
collective copper wire screens 
626 7870-5 Polymeric insulated aerial bundled cables rated 
600/1000 V for overhead distribution 
604 7870-6 Polymeric insulated cables for generation rated at 
600/1000 V and 1900/3300 V 
622 7870-7 Polymeric insulated cables for generation rated from 
3800/6600 V up to 19 000/33 000 V 
9.6.2 Optical communication cables 
For communication systems and their evolution to be effective, standardization must 
be at an international level. Optical fibre and cable standardization in lEC started in 
1979. The ENs which have been published generally use lEC standards as a starting 
point but they incorporate any special requirements for sale within the EU where 
European Directives may apply. 
Table 9.2 summarizes the main standards in the areas of optical fibres, optical 
cables, connectors, connector interfaces and test and measurement procedures for 
interconnecting devices. 
Table 9.2 International and national standards for optical fibres, optical cables, connectors, connector 
interfaces and test and measurement procedures for interconnecting devices 
lEC EN BS Subject 
60793-1 
60793-2 
60794-1-1 
60794-1-2 
60794-2 
60794-3 
60794-4 
60869 
60874 
60875 
60876 
61202 
61274 
61300 
50174 
60793-1 
60793-2 
60794-1-1 
60794-1-2 
60794-2 
60794-3 
60794-4 
60869 
60874 
60875 
60876 
61202 
61274 
61300 
EN 50174 
EN 60793-1 
EN 60793-2 
EN 60794-1-1 
EN 60794-1-2 
EN 60794-2 
EN 60794-3 
EN 60794-4 
EN 60869 
EN 60874 
EN 60875 
EN 60876 
EN 61202 
EN 61274 
EN 61300 
Information technology - Cabling installation 
Optical fibres: Measurement and test procedures 
Optical fibres: Product specifications 
Optical fibre cables: Generic specification 
Optical fibre cables: Basic test procedures 
Indoor optical fibre cables 
Outdoor optical fibre cables 
Aerial optical cables along overhead lines 
Fibre optic attenuators 
Connectors for optical fibres and cables 
Fibre optic branching devices 
Fibre optic spatial switches 
Fibre optic isolators 
Fibre optic adaptors 
Fibre optic interconnecting devices and passive 
components test and measurement procedures 
(contd) 
278 Wires and cables 
Table 9.2 (contd) 
lEC EN BS Subject 
61314 
61753 
61754 
61977 
61978 
62005 
62077 
62099 
62134 
61314 
61753 
61754 
61977 
61978 
62005 
62077 
62099 
62134 
181000 to 
181104 
186000 to 
186310 
187103 
187105 
EN 61314 
EN 61753 
EN 61754 
EN 61977 
EN 61978 
EN 62005 
EN 62077 
EN 62099 
EN 2134 
EN 181000 to 
181104 
EN 186000 to 
186310 
EN 187103 
EN 187105 
Fibre optic fanouts 
Fibre optic interconnecting devices and passive 
components performance standards 
Fibre optic connector Interfaces 
Fibre optic filters 
Fibre optic compensators 
Reliability of fibre optic interconnecting devices and 
passive components 
Fibre optic circulators 
Fibre optic wavelength switches 
Fibre optic enclosures 
Fibre optic branching devices 
Connector sets for optical fibres and cables 
Optical fibre cables for indoor applications 
Single-mode optical cables for duct or 
buried installation 
References 
9A. Moore, G.F. Electric Cables Handbook (3rd edn), Blackwell Scientific Publications 
Ltd, 1997. 
9B. Heinhold, L. Power Cables and their Application (3rd revision), Siemens AG, 1990.